Flexible high performance microbolometer detector material fabricated via controlled ion beam sputter deposition process

ABSTRACT

A microbolometer film material VOx having a value such that the thermal coefficient of resistance is between 0.005 and 0.05. The film material may be formed on a wafer. The VOx material properties can be changed or modified by controlling certain parameters in the ion beat sputter deposition environment. There is sufficient control of the oxidation process to permit non-stoichometric formation of VOx films. The process is a low temperature process (less than 100 degrees C.). Argon is used for sputtering a target of vanadium in an environment wherein the oxygen level is controlled to determine the x of VOx. The thickness of the film is controlled by the time of the deposition. Other layers may be deposited as needed to form pixels for a bolometer array.

This is a divisional of application Ser. No. 08/770,894, filed Dec. 31,1996, now U.S. Pat. No. 6,322,670 which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention pertains to microbolometer sensors andparticularly to detector material for microbolometers. Moreparticularly, the invention pertains to a particular detector materialwhich is fabrication from a special ion beam sputter deposition process.The U.S. Government has certain rights in the present invention.

A major factor in the sensitivity of a bolometer is the TCR (thermalcoefficient of resistance) of the detector material. The overall NETDsensitivity of the bolometer also depends on the noise level. Previousbolometer materials are typically high TCR metals with a TCR in therange from 0.003 to 0.004. These materials have low noise but also havelow TCR. Since the metals are reflectors, they also degrade theabsorbance properties of the detector. Materials which undergo a phasetransitions (i.e., Mott transition) can have a very high TCR's in thetransition region but can suffer from a number of problems. First, thelatent heat accompanying the phase change for these materials maysignificantly decrease the sensitivity of the detector. Second, mostswitching material can be produced in only one form without additionaldoping, which defines the material resistance and TCR. Further, thetemperature range over which the transition occurs is typically verysmall requiring tight temperature control of the operation. Finally, thefilms must be produced in crystalline form which requires hightemperature depositions.

SUMMARY OF THE INVENTION

The present invention is peculiar vanadium oxide (VOx/ABx) (i.e.,VO_(x)/AB_(x)) detector material and process that is used to make thatmaterial. The x of VOx is a value fitting for the pixel being sputterdeposited by the present process and is not necessarily a specific digitsuch as “2”, but may be between 1 and 2.5. That material is deposited aspart of a pixel for a high performance microbolometer. The material isdeposited by an ion beam sputtering with control of the depositionprocess leading to a flexible detector process for microbolometerdetectors. These detector materials have optical, electrical, andthermal properties compatible with high performance detectors but whichcan be readily modified to suit individual requirements of an arraydesign.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 reveals a TCR comparison of various materials, including VOx(ABx).

FIG. 2 is a graph showing electrical characteristics of a high TCR VOxdetector film.

FIG. 3 exhibits determination of Vox resistance by control of thedeposition environment.

FIGS. 4a and 4 b are a schematic of the deposition system.

FIG. 5 is a schematic of the deposition process flow.

FIGS. 6a and 6 b show several stages of the deposited wafer.

DESCRIPTION OF THE PREFERRED EMBODIMENT

VOx deposited by a controlled ion beam sputtering process has producedbolometer detector films with a wide range of performance propertieswhich has led to a flexible detector manufacturing process. Some of theadvantages of the present detector film materials of the invention,which is the class of these materials and the processing used to makethese materials, are noted. Bolometer film materials with high TCR'sranging from 0.005 to >0.03, such as 0.05, have been demonstrated. For agiven resistivity, these materials are higher performing inmicrobolometers than any known material except for a few crystallinematerials (which have not been produced on pixels), as shown in FIG. 1.FIG. 1 is a graph showing a comparison of the TCR versus conductivity ofvarious materials relative to ABx (e.g., VOx). This class of materialshas a well behaved relationship between resistance and TCR given by therelationship TCR=A+B*log(rho) (where “rho” is the film resistivity)which, for example, may be TCR=0.03+0.01*log(rho). A wide range of pixelresistance and TCR properties are possible by using differentresistivity materials (obtained by slight modifications to thedeposition process) combined with different detector patterns andthicknesses. These electrical properties for any particular VOx film arewell behaved and characterized over a wide range of temperatures and arenot limited to a narrow transition region, as indicated by FIG. 2. Forany particular VOx film, resistance is given by LnR=A+(B/T) and the TCRwhich is defined as $\frac{1}{R}\frac{R}{T}$

is therefore TCR=B/T². FIG. 2 exhibits the electrical characteristics ofa high TCR VOx detector film. These characteristics reveal a materialhaving a well-behaved operation over a wide temperature range.Resistance levels are in the proper range to permit high currentbolometer operation for optimal responsivity. The films are amorphousand exhibit no latent heat effects unlike the phase transition effectsin VO2. The films are stable after annealing if not taken to a highertemperature. The resistance change on annealing is well-defined and canbe corrected for by changes to the initial deposition conditions. 1/fnoise levels are defined by$V_{noise} = {V_{bias}{\sqrt{\frac{k}{f}}.}}$

k values as low as 10⁻¹² to 10⁻¹⁴ make 1/f noise contributions to totalnoise very small. Noise levels are close to Johnson noise limitedvalues.

The optical properties of VOX are compatible with high absorbance in thedetector. The thermal mass of VOx, the thermal property of importance,is comparable to the major pixel material, Si3N4 (i.e., Si₃N₄). TheseVOx films have a high TCR over a range of thicknesses from as low as afew hundred Angstroms to as thick as 1500 Angstroms. This material ofthe films is compatible with microbolometer properties. The Vox materialproperties can readily be modified by a simple change in the ion beamsputter deposition environment of the process of the present invention,as revealed by FIG. 3. FIG. 3 is a graph that shows determination of Voxresistance by control of the ion beam sputter deposition environmentsuch as the gas control level. The present ion beam sputtering providessufficient control of the oxidation process to permit non-stoichiometricformation of Vox films. In other reactive deposition techniques, theoxidation process tends to proceed to completion forming onlystoichiometric material. The ion beam sputter deposition is a lowertemperature deposition process. This means that added flexibility in thepatterning of VOx films can be achieved via liftoff processing whichentails the use of photoresist during deposition.

The method of the present invention is a process 18 of FIG. 5, performedin conjunction with deposition system 10 of FIG. 4a, which is capable ofmaking the above-noted VOx material. Circular silicon wafers 11 withsubstrates containing electronic circuits and pixel lower layers 12 (seeFIG. 6a) which are coated with an approximately 5000 Angstrom Si3N4layer 13, are loaded into a five-wafer carousel 19 (of FIGS. 4a and 4 b)through port 15 of deposition apparatus 10. Also, photoresist 74 (seeFIG. 6a) may be on wafer 11 which defines pixels. At present, each wafer11 has a diameter 14 of four inches. Wafer 11 loading is step 16 ofprocess 18 flowchart. System 10 is first calibrated according tocalibration step 20 of process 18, which involves pumping the systemdown to 2×10⁻⁷ Torr pressure. The vacuum system is capable of pumping tobase pressure less than 1×10⁻⁷ Torr, with throughput to an eight inchtwo-stage cryopump 23 via valve 24. System 10 is pumped down by openingvalve 21 which is left open during the process. System 10 may be warmedup at step 25 with lamps 45. A residual gas analyzer (RGA) 22 is warmedup at step 26 before the process calibration. RGA 22 is connected tochamber 27 and has a quadrupole probe which has an electron multiplier(EM) detector in a turbo-pumped sampling manifold to allow for operationin the 10⁻⁷ Torr pressure environment. RGA 22 may be connected to eitherupper chamber 31 or lower chamber 27. RGA 22 has an analog output.

Silicon wafers 11 may be anything with active electronics thereon, suchas CMOS or bipolar devices 73 of FIGS. 6a and 6 b, or other kinds ofdetector components up to the Si3N4 layer 13, photoresist 74 and/or VOxlayer 79 deposition. RGA 22 valve 28 is opened and argon flow is set toapproximately 3 scc/m at stage 29 of process 18. RGA 22 is calibratedvia an adjustment of its gain to get a standard RGA argon reading atstep 30. RGA 22 is a closed source device. It is connected to chamber 27in FIG. 4a. The output of RGA 22 detects a spectrum of species of gasconstituents and the detected gases are identified. RGA 22 is pumped outso that the pressure in RGA 22 is lower than chamber 27, and the ionspecies have to go through a small orifice at valve 28. RGA 22 is madeby UTI Inc. With RGA 22 valve open, the argon gas flow reading is setfor calibrating RGA 22 to a known argon peak level.

At step 32, the argon gas flow is set for gun 33 and hollow cathodeneutralizer (HCN) 34. Gun 33 uses argon as a sputtering gas. Ion beam 35is neutralized by beam 40 of HCN 34. The ion gun is from Ion Tech Inc.

A target on target holder 36 is positioned relative to gun 33 beam 35 sothat vanadium is selected as a target material 46. The other side oftarget holder has SiO2 as a selectable target material 47. The targetmaterial is set at a 45 to 55 degree angle relative to the direction ofbeam 35. The center of the target material is at a distance of 7 and ⅝inches from the beam 35 exit of gun 33. Argon gas flow to gun 33 is setwith MFC 37 to a 2.5 scc/m operating level, and to HCN 34 set with MFC39 to a 3.5 scc/m operating level. Xenon (Xe) may also be used in placeof Argon (Ar). Cooling water from source 41 is turned on to gun 33 vialine 42 and to target holder 36 via line 43. At step 38, an ion gunpower supply 44 is turned on and the initial gun start-up parameters areset to 20 mA at 1 kV. The ion gun source is turned on and the sourcestabilizes. Beam 35 of ion gun 33 is turned on. The power supplyvoltages are adjusted. There is plasma in gun 33 and ion beam 35 isgenerated or accelerated through grids of gun 33.

At next step 48, target material 46 is pre-sputtered with no oxygen.Pre-heat system quartz lamps 45 are turned off if on. Target material 46is pre-sputtered for 240 seconds at the low power of 20 mA at 1 kV.During the next 240 seconds, target 46 is presputtered at the mediumpower of 35 mA at 1.5 kV. Presputter continues for the next 120 secondsat the high power of 50 mA at 2 kV, which concludes system calibrationstage 20. The presputter without oxygen is for cleaning target 46 forten minutes or so.

The following stage 50 begins with step 49 of presputter with an oxygenramp to condition target 46 to a desired RGA 22 level. The oxygen goesup by little steps and then to larger steps as one measures the oxygenwithin system 18. This is not cleaning target 46 but conditioning target46 to the desired RGA 22 level. One increases the oxygen to get acharacter profile for film 79 based on past experience and RGA 22 is setat an arbitrary level called y. The beam 35 power is kept at 50 mA at 2kV and controller 53 of MFC 51 sets the oxygen flow to chamber 27 viatube 52, to 0.5 scc/m for 90 seconds. Then the oxygen flow is increasedby 0.1 scc/m for 90 seconds. The latter is repeated until the 32 AMUpartial pressure increase is equal to or greater than ten times over theprevious partial pressure. The operating level setpoint is based on thepartial pressure rise of the previous step which is about midpoint ofthe last 32 AMU partial pressure increase. The precise location of theoperating setpoint determines the resistance and TCR. The 32 AMU partialpressure setpoint is entered into controller 53, which determines thelevel of oxygen flow for VOx film 79 deposition. One increases the flowof oxygen in an incremental way until O₂ is at a point where the RGAoutputs an O₂ signal. One measures the mass and monitors the RGA O₂. Iongun 33 is run at a set level with a fixed voltage and current.Monitoring of RGA 22 of 32 AMU is done at controller 53 where the flowis adjusted to achieve the starting level. A computer processor 56 maybe used at step 59 to monitor RGA 22 and adjust the oxygen flow viacontroller 53 to achieve starting condition or level.

At step 54, rotation of wafer-substrate 11 is started. A control loop isstarted with a presputter for 300 seconds at the setpoint of controller53. A shutter 55 is opened at step 58 after system 10 has stabilized. Atimer 57 is started with the time determined by a desired thickness, ofwhich the deposition rate is approximately at 25 Angstroms per second.RGA 22 may be monitored and oxygen flow adjusted at step 60 duringdeposition step 61. The center of sputtered target 46 with its surfaceat 45 to 55 degrees relative to and 12 inches from the to-be-depositedsurface of wafer 11, is aligned at one inch from the center of wafer 11.After the desired thickness is achieved, then shutter 55 is closed atstep 62. Then carousel 19 is turned at step 63 for the next wafer 11 tobe coated, the control loop starts with the presputter at step 49 ofdeposition stage 20, and goes through the same steps of the process forthe previous wafer 11 deposition. After all the substrates 11 ofcarousel are deposited, the control loop of stage 20 is stopped. Theoxygen MFC 51 is set to zero scc/m, RGA 22 sample valve 28 is closed,ion beam 35 is turned off or to stand-by mode, and RGA 22 is turned off.

Process 18 is a low temperature process which does not go over 100degrees C. which would cause photoresist 74 to harden. Typically thisprocess is performed at about 80 degrees C. or less.

Process 18 for wafers 11 moves on to stage 64 for SiO2 deposition. Atstep 65, target holder 36 is rotated so that the surface of target 47will be at a 45 degree angle relative to the direction at the center ofion beam 35 when it is turned on, and the oxygen flow is set to 2.0scc/m at MFC 51. First wafer 11 is rotated in by carousel 19 as step 66.Ion gun 33 is turned on at 50 mA at 2 kV to presputter target 47 for 300seconds. Shutter 55 is opened and timer 57 is started. Timer 57 is setto a time period to attain a desired thickness of SiO2 on wafer 11 at adeposition rate of 0.33 Angstrom per second at step 67. Then shutter 55is closed. Next wafer 11 is rotated to by carousel 19 for SiO2deposition at step 68, and steps for depositing SiO2 on previous wafer11 are followed. After the last wafer 11 is coated with deposition ofSiO2, the system 10 is shut down at step 69 of stage 64. Ion beam 35 isturned off, MFC 51 is set to 0.0 scc/m, ion beam source 33 is turnedoff, and substrate rotator 70 and carousel 19 are turned off. The oneshould wait and let system 10 cool down for 45 minutes. After cool-down,Hivac valve 24 is closed, and system 10 is vented with dry N2 fromsupply 71 through valve 72. One may open system 10 when it is atatmospheric pressure and remove wafers 11. To start process 18 over withanother set of substrates or wafers 11, one introduces wafers 11 intoupper chamber 31 through port 15 and close system 10. Quartz lamps 45are turned on to a preset level to yield 80 degree C. temperatures insystem 10. N2 vent gas valve 72 is turned off and MFC 37 and MFC 39 areset to 0.0 scc/m. Next pump down with cyropump 23 and open Hivac valve24. Check for leaks and go through process 18 as indicated above. Thevalues of the parameters and settings of the above-noted embodiments areby example only, but could vary from case to case.

From wafer 11, processed by system 10, is made microbolometer pixels 77of FIGS. 6a and 6 b. On wafer 11, prior to system 10 depositions of VOxand SiO2, may be Si substrate 12 covered with a device layer 73. Onlayer 73 is pixel Si3N4 material layer 13. Pixels 77 are defined with aphotoresist mask 74. Then, VOx layer 79 and SiO2 layer 75 arerespectively deposited on wafer 11 as indicated above and revealed inFIG. 6a. The next step is to chemically remove photoresist 74, and aportion of layers 79 and 75 formed on and over photoresist layer islikewise removed, resulting in pixel 77 as shown in FIG. 6b. A via orhole 78 is etched through layer 75 for electrical contact which is madewith a nichrome (NiCr) strip 76 formed by depositing and patterning on asmall portion of pixel 77. NiCr 76 forms the contact to the VOx portionof pixel 77. VOx layer 79 covers a major portion of pixel 77. Anadditional Si3N4 layer may be formed on pixel 77 of FIG. 6b for moreprotection. Layer 75 is about 200 Angstroms. If layer 75 were not put onprior to placement of contact 76, then the VOx portion of the pixelwould be degraded due to the electrical degradation of VOx duringpresputter of the film prior to Si3N4 and subsequent NiCr deposition ofstrip 76. Layer 79 may range from 200-2000 Angstroms depending on thedesired TCR. Layer 13 is about 500 Angstroms. But if an Si3N4 layer 80is formed on layer 75, then NiCr strip would go through layer 80 andlayer 75 to contact VOx layer 79.

We claim:
 1. A VOx material comprising vanadium and oxygen forming respective portions of the VOx material, wherein x is a value selected to determine a thermal coefficient of resistance (TCR) of between 0.005° C.⁻¹ and 0.05° C⁻¹.
 2. The VOx material of claim 1, wherein the relationship between VOx material resistance and TCR is provided by the relationship TCR=A+B*log(rho), where A and B are numerical constants.
 3. The VOx material of claim 2, wherein: A is between 0.005 and 0.05; and B is between 0.1 and 0.001.
 4. A VOx material comprising vanadium and oxygen, the material having a thermal coefficient of resistance (TCR) of between 0.005 and 0.05° C.⁻¹ and a log of conductivity between zero and three.
 5. The VOx material of claim 4, wherein the TCR is between 0.005 and 0.03° C.⁻¹.
 6. The VOx material of claim 4, the material further having a resistivity (rho), wherein the relationship between TCR and resistivity (rho) is generally defined by the equation TCR=A+B*log(rho), where A and B are numerical constants.
 7. The VOx material of claim 6, wherein: A is between 0.005 and 0.05; and B is between 0.001 and 0.1. 